The invention relates generally to fuel cell systems, and more particularly to methods and systems of starting fuel cell systems.
To start a fuel cell system, reactants must be available at the cell's active area such that electric current can be generated. This is typically the main function of the start sequence if the system had been off or in a standby state. For vehicle applications, it is desirable to complete the start sequence so that the driver is able to request full power soon after the start request is made. In addition, when the fuel cell system is exiting from a standby mode, it is desirable that the system is able to run at capacity soon after the wake up request is made.
To prepare a fuel cell system such to operate at full capacity, the start sequence will typically include an anode flush function and an anode fill function. The flush function removes non-reactants from the cell flow fields. The fill function increases the hydrogen concentration at the active sites to the desired level. The fill function typically accounts for the majority of the time needed to start for anode side functions. This is due to a combination of exhaust emissions requirements and non uniform flow characteristics within the stack plumbing and stack. In practice with a non-ideal header purge, the anode flush function will purge non-reactants to a level as illustrated in FIG. 1. Even an ideal flush may require additional fill time to get uniform hydrogen to all cells. This would typically be random cells based on cell flow resistance variability.
At the end of the anode flush after a non-ideal header purge, which typically occurs during the start sequence, the anode will still contain a percentage of non-reactant gas in the top cells. If full current is requested at that time, the cells that are only partially filled with hydrogen will not be able to support the current, causing a sharp drop in cell voltage. To prevent this from occurring, a voltage stabilization function is employed to fill the remainder of the stack with hydrogen. A lower hydrogen flow rate is used for this function, and the flow rate is controllable so that the emissions requirement is not violated. However, this is done at a cost of increased start length. The typical time to perform the anode flush is about 1 second, while the additional anode fill and voltage stabilization can take up to 4 seconds.
Therefore, there is a need for an improved start-up method for a fuel cell.